1. Field of the Invention
The invention relates to an arrangement for providing and feeding transfer material for a thermographic process for producing printing plates on a plate cylinder. The arrangement includes a laser image-setting head which can interact with a thermal transfer ribbon, and means for guiding and positioning the thermal transfer ribbon with respect to the plate cylinder. The laser image-setting head and thermal transfer ribbon can be moved along the plate cylinder by a traversing unit.
2. Description of the Related Art
U.S. Pat. No. 5,601,022 discloses such an arrangement for providing and feeding transfer material for a thermographic process for producing printing plates, that is to say for setting an image on a printing plate in the printing machine. An image-setting system of this type for digitally written and re-erasable offset printing plates operates on the external drum principle, a narrow ink ribbon, similar to a typewriter ink ribbon, being lead past the rapidly rotating printing cylinder while a laser beam transfers the layer from the ink ribbon to the printing cylinder by means of heat. This produces the ink-carrying parts of the image.
Changing from one job to the next is a three-stage process: image-setting, fixing and image removal (erasure). The most important components of the image-setting procedure are the laser image-setting head, the thermal transfer ribbon and the plate cylinder.
The laser image-setting head receives the digital image data from the control desk. The polymer-coated thermal transfer ribbon is brought into close contact with the surface of the plate cylinder, which is preferably constructed as a printing sleeve. The laser image-setting head, with up to more than 200 channels, heats the thermal transfer ribbon pixel by pixel in accordance with the image information to be transferred. The laser and ribbon move transversely over the plate cylinder and therefore, in a spiral or helix, produce the printing image on the surface of the plate cylinder with high resolution.
The polymer applied is generally ink-carrying, the naked cylinder surface, that is to say the naked sleeve, is water-carrying.
As already shown by DE 198 11 031 A1, the thermal transfer ribbon is preferably located in an easily interchangeable cassette housing. After the passage of a complete track length, the ribbon is wound back and, if necessary, a further, unused track can be set up toward the laser image-setting head, so that a further image-setting process can be carried out. When such cassette housings are used, friction generally occurs between the thermal transfer ribbon and guide parts belonging to the cassette housing, so that, firstly, only limited ribbon speeds are possible and, in addition, the ribbon itself wears, or the functional layer is damaged.
In addition, the previously disclosed arrangement, referred to below as a ribbon station, is also restricted to setting the ribbon on and off with respect to the plate cylinder, and in this way only offers wear-afflicted ribbon guidance, depending on the cassette housing provided.
It is therefore an object of the invention to develop a generic arrangement, that is to say a ribbon station, in such a way that wear-free and therefore fast ribbon guidance can be performed and, after accommodating a cassette housing suitable for it, together with the thermal transfer ribbon ready prepared therein, the ribbon station permits the automation of the movement sequences for producing a printing plate on a plate cylinder.
The intention, therefore, is to apply a more developed cassette housing, in which the specially prepared thermal transfer ribbon can be transported in a manner completely decoupled from the cassette housing, as described in DE 100 23 319, to which U.S. application Ser. No. 09/851,919 filed on May 9, 2001 corresponds. This U.S. application is incorporated herein by reference.
Provided on the side faces of the cassette housing described there are holes, through which the hubs of the spools, that is to say at least one unwind and one winding roller, project from the housing. These hubs have one or more grooves to connect them firmly to suitable drivers so as to rotate therewith. The hubs have play within the holes, so that they can be positioned freely in the radial direction with respect to the housing.
It is viewed as particularly advantageous that the cassette housing does not have to have any ribbon guiding function and can therefore be constructed particularly simply. Neither bearings for the spools nor ribbon guide rollers are provided in the cassette housing instead, a transport safeguard is provided which prevents inadvertent unwinding but permits the thermal transfer ribbon to be wound in order to tension it, is preferably an integral constituent of the cassette housing and can be released both manually and under automatic control.
The object set above is achieved in a generic arrangement for providing and feeding transfer material for a thermographic process for producing printing plates on a plate cylinder. The invention provides a first structural unit, which comprises the laser image-setting head, and a second structural unit, which comprises a loading compartment to accommodate the thermal transfer ribbon, specially prepared in the cassette housing. Means for guiding and positioning the ribbon are motor-driven and designed to be self-centering, in such a way that to produce a printing plate, the second structural unit can be brought over the first structural unit in the manner of a telescope by means of a moving device, and therefore the thermal transfer ribbon can be brought to the laser image-setting head, and can be positioned freely to accommodate the thermal transfer ribbon. The thermal transfer ribbon inserted into the loading compartment can be guided without contact with respect to the cassette housing by the self-centering means and, to produce a printing plate, can be positioned with respect to the plate cylinder.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.